An isolated footing has one job: spread the column load over enough soil area that the ground can carry it, without the footing itself breaking. The design walks through five checks in a fixed order. Here is the logic, followed by a complete worked example.
The five steps of isolated footing design
- Size the base area from the service (unfactored) load and the safe bearing capacity (SBC) of the soil.
- Compute the net factored upward soil pressure for structural design.
- Fix the depth — usually governed by shear (one-way or punching), not bending.
- Design the bending reinforcement at the column face.
- Check development length and detail the steel both ways.
Worked example: the numbers
Take a typical interior column of a G+2 house:
Bending at the column face
Bending alone would allow a very thin footing — but shear almost always governs, which is why real footings are much deeper than the bending check suggests.
Shear: the checks that set the depth
- One-way (beam) shear is checked on a section at distance d from the column face; the concrete’s design shear strength τc (from IS 456 Table 19, a function of grade and steel percentage) must exceed the applied shear stress.
- Two-way (punching) shear is checked on a perimeter at d/2 around the column; the allowable stress is ks·0.25·√fck. Punching is usually the critical check for square footings under heavy columns.
Reinforcement
What this example deliberately leaves out
A real design also considers: moments and horizontal loads from the frame analysis, biaxial cases, water table (which reduces effective bearing), footings at different levels, proximity to boundaries (eccentric footings), and settlement. This is exactly the judgement layer a licensed structural engineer must apply to your specific site — a worked example, or any software output, is a starting point, never a construction document.
Frequently asked questions
What is safe bearing capacity (SBC)?
The pressure the soil can safely carry without shear failure or excessive settlement, determined from a soil investigation. Typical values range from under 100 kN/m² for soft clays to 300+ kN/m² for dense sands and weathered rock.
Why is the footing checked at “d from the face” and “d/2 from the face”?
These are the code-defined critical sections: one-way shear cracks form on an inclined plane starting about an effective depth (d) from the column face, while punching failure forms a truncated cone around the column, checked on a perimeter at d/2.
What minimum depth and cover should a footing have?
Footings generally use at least 50 mm cover against soil (on lean concrete/PCC) and a practical minimum overall thickness of about 300 mm; the governing depth comes from the shear checks.